There is an increasing demand for environmentally friendly hydrocarbons and clean-burning high performance fuels, such as distillate fuels like diesel and jet fuels. Distillate fuels typically contain paraffins, naphthenes, and aromatics having greater than 9 carbon atoms. For fuel quality parameters such as cetane, gravity and emissions, paraffins are the most desirable components, followed by naphthenes, followed by aromatics. The least desirable are multi-ring aromatic compounds. While various refinery processes produce distillate fuels, these processes are typically limited in their capability to produce high quality distillate fuel and/or high yields of distillate fuel. For example, conventional hydrogenation processes saturate aromatic rings to naphthenes, thereby increasing the cetane number, and increasing the API gravity (lower density). The disadvantage of hydrogenation alone is that naphthenes have generally lower cetane values and are more dense than paraffins having substantially the same number of carbon atoms. The greater density of naphthenes results in reduced volume of the distillate fuel blend relative to a composition containing similar concentrations of paraffins instead of naphthenes. Similarly, multi-ring naphthenes are generally more dense and have lower cetane values than single-ring naphthenes having substantially the same number of carbon atoms. In addition, naphthenes can be converted to aromatics via oxidation reactions. Since combustion of naphthenes in fuels occurs under oxidizing conditions, there is the potential for naphthenes to revert to aromatics under combustion conditions, thus reducing fuel quality and increasing emissions of undesirable compounds. Consequently, it is desirable to ring open naphthenes to produce the corresponding paraffins. Conversion of naphthenes to paraffins, using noble metal catalysts, is known to produce fuels with higher cetane number and higher API gravity. A significant problem associated with the use of noble metal catalysts for ring opening is their deactivation in the presence of sulfur. Consequently, it would be advantageous to have a process which could integrate hydrodesulfurization with ring opening.
Hydrodesulfurization is one of the fundamental processes of the refining and petrochemical industries. The removal of feed sulfur by conversion to hydrogen sulfide is typically achieved by reaction with hydrogen over non-noble metal sulfides, especially those of Co/Mo and Ni/Mo, at fairly severe temperatures and pressures to meet product quality specifications, or to supply a desulfurized stream to a subsequent sulfur sensitive process. The latter is a particularly important objective because some processes are carried out over catalysts which are extremely sensitive to poisoning by sulfur. This sulfur sensitivity is sometimes sufficiently acute as to require a substantially sulfur free feed. In other cases environmental considerations and mandates drive product quality specifications to very low sulfur levels.
There is a well established hierarchy in the ease of sulfur removal from the various organosulfur compounds common to refinery and chemical streams. Simple aliphatic, naphthenic, and aromatic mercaptans, sulfides, di- and polysulfides and the like surrender their sulfur more readily than the class of heterocyclic sulfur compounds comprised of thiophene and its higher homologs and analogs. Within the generic thiophenic class, desulfurization reactivity decreases with increasing molecular structure and complexity. While simple thiophenes represent the more labile sulfur types the other extreme, which is sometimes referred to as "hard sulfur" or "refractory sulfur," is represented by the derivatives of dibenzothiophene, especially those mono- and di-substituted and condensed ring dibenzothiophenes bearing substituents on the carbons beta to the sulfur atom. These highly refractory sulfur heterocycles resist desulfurization as a consequence of steric inhibition precluding the requisite catalyst-substrate interaction. For this reason, these materials survive traditional desulfurization processes and poison subsequent processes whose operability is dependent upon a sulfur sensitive catalyst. Destruction of these "hard sulfur" types can be accomplished under relatively severe process conditions, but this may prove to be economically undesirable owing to the onset of harmful side reactions leading to feed and/or product degradation. Also, the level of investment and operating costs required to drive the severe process conditions may be too great for the required sulfur specification.
A recent review (M. J. Girgis and B. C. Gates, Ind. Eng. Chem., 1991, 30, 2021) addresses the fate of various thiophenic types at reaction conditions employed industrially, e.g., 340-425.degree. C. (644-799.degree. F.), 825-2550 psig. For dibenzothiophenes the substitution of a methyl group into the 4-position or into the 4- and 6-positions decreases the desulfurization activity by an order of magnitude. These authors state, "These methyl-substituted dibenzothiophenes are now recognized as the organosulfur compounds that are most slowly converted in the HDS of heavy fossil fuels. One of the challenges for future technology is to find catalysts and processes to desulfurize them."
M. Houalla et al, J. Catal., 61, 523 (1980) disclose activity debits of 1-10 orders of magnitude for similarly substituted dibenzothiophenes under similar hydrodesulfurization conditions. While the literature addresses methyl substituted dibenzothiophenes, it is apparent that substitution with alkyl substituents greater than methyl , e.g., 4,6-diethyldibenzothiophene, would intensify the refractory nature of these sulfur compounds. Condensed ring aromatic substituents incorporating the 3,4 and/or 6,7 carbons would exert a similar negative influence. Similar results are described by Lamure-Meille et al, Applied Catalysis A: General, 131, 143, (1995) based on similar substrates.
Mochida et al, Catalysis Today, 29, 185 (1996) address the deep desulfurization of diesel fuels from the perspective of process and catalyst designs aimed at the conversion of the refractory sulfur types, which "are hardly desulfurized in the conventional HDS process." These authors optimize their process to a product sulfur level of 0.016 wt. %, which reflects the inability of an idealized system to drive the conversion of the most resistant sulfur molecules to extinction. Vasudevan et al, Catalysis Reviews, 38, 161(1996) in a discussion of deep HDS catalysis report that while Pt and Ir catalysts were initially highly active on refractory sulfur species, both catalysts deactivated with time on oil.
In light of the above, there is a need for a desulfurization/ring-opening process capable of converting feeds bearing the refractory, condensed ring sulfur heterocycles at relatively mild process conditions to streams containing substantially no sulfur. Such streams will not deactivate the ring opening catalyst.